The invention is related to the field of resonant wireless power (RWP), and in particular to a RWP driver with adjustable power output.
Wireless power (WP) transfer systems use the mutual inductance between two magnetic coils to transfer power through magnetic induction. These systems are commonly classified as either “inductive” or “resonant”. In a purely inductive wireless power transfer system, the source coil, which functions as the primary winding of a transformer, is driven by a voltage or current source. The receive coil, which functions as the secondary winding, is connected to a bridge rectifier, either directly or through an ac-coupling capacitor. The voltages and currents in the two windings can be determined by the relations commonly used to describe transformers.
In a resonant wireless power (RWP) transfer system, the source and receiver coils are connected to capacitors to form electrical resonators. From a circuit-design standpoint, the function of the capacitors is to cancel some of the reactive impedance of the inductors, allowing more power to be transferred at a given voltage. The impedance of the inductors and capacitors varies in opposite directions with operating frequency, so the cancellation is only effective over a small range of frequencies. In other words, resonant wireless power systems utilize circuits tuned to a specific frequency at which power is to be transferred. They typically do not allow power transfer at other frequencies.
In order to operate a RWP system at high efficiency over a wide range of loading conditions, the power output from the wireless power source must be adjustable. Operating the source at too low of a power level may result in insufficient power to supply the receiver devices. Operating it at too high of a power level may result in excess circulating currents, causing wasted energy.
For MHz-frequency RWP sources, a Class-E amplifier is often used to convert dc power to ac. The Class-E amplifier is only efficient at one particular duty cycle, so it is not possible to use duty cycle control to modulate power. Nor is it typically possible to vary frequency, as the receiver devices are all tuned to a specific frequency. The best-known methodology for adjusting power level is to introduce a dc-dc converter between the input dc power supply and the Class-E amplifier. However, this dc-dc converter will introduce some efficiency loss as well as adding complexity to the design of the source electronics.